1. Field of the Invention
The present invention relates to exhaust systems for two stroke engines and more particularly to those exhaust systems which are tuned or incorporate expansion chambers. The invention relates specifically to a tuned exhaust system which incorporates a uniquely oriented converging cone.
2. Description of the Related Art
Internal combustion engines, particularly the two stroke variety, have long been in existence. However, the use of two stroke engines with high power ratings has been restricted to a very narrow r.p.m. range because the level of performance tends to drop rapidly in the lower r.p.m. values. This drop in performance is a result of the dynamics of the gases flowing through the exhaust system. The dynamics of the gases have a strong influence on gas exchange during the scavenging phase and ultimately on engine performance.
It is well known that an expansion chamber, utilizing the sonic energy in the exhaust stream, can be used to provide and shape a pressure pulse that is returned to the exhaust port of a two stroke engine. When properly applied, an expansion chamber can improve the performance and scavenging effects in a two stroke engine. Two stroke engines having an exhaust system incorporating an expansion chamber typically contain a first pipe of constant cross-sectional dimension extending from the exhaust port. The first pipe is followed by a diffuser section having a divergent wall which defines a progressively expanding cross-section. The diffuser section is used for generating a negative pressure wave, or vacuum, for drawing out exhaust gases from the engine. The diffuser ultimately leads to a conical section, or baffle, having a convergent wall which defines a progressively decreasing cross-section. The conical section is used to generate a positive pressure wave directed back at the exhaust port. A short piece of pipe is typically situated aft of the conical section in order to allow the exhaust gases to exit the expansion chamber.
When the exhaust port is opened, a pressure pulse travels outward. A negative pressure wave is generated in the diffuser section as the outward pressure pulse travels through the diffuser. This negative pressure wave increases the scavenging of combusted fuel from the engine. During passage of the outward pressure pulse through the converging or baffle section, a positive pressure wave is generated. The positive pressure wave is reflected back to the cylinder, while the exhaust port is still open, and increases the pressure therein. This positive pressure wave also causes any non-combusted fuel and air mixture, which has entered the exhaust pipe from the cylinder during the scavenging phase, to return into the cylinder, thus further charging the cylinder.
The shape of the pressure waves generated by the expansion chamber, both negative and positive, is determined primarily by the rate of change of cross sectional area of the expansion chamber. In essence, the shape of the pressure wave is proportional to the square of the primary dimension of the expansion chamber; diameter of a circle, length of a side of a square, etc. Therefore, in a conventional design, the reflected pressure wave from the converging cone increases rapidly as the outward pressure pulse enters the cone and more slowly as the outward pulse travels the length of the cone. The inverse is true in the inverted cone. The reflected positive pressure wave increases more rapidly as the outward pressure pulse approaches the end of the cone. Therefore, the reflected positive pressure wave has a sharper cut-off with the inverted cone, which provides for more of the energy contained within it to be captured by the closing of the exhaust port.
The prior art provides various systems for improving the operation of a two stroke engine. For example, U.S. Pat. No. 4,226,298 issued on Oct. 7, 1980 to Bancel et al. discloses an exhaust system for internal combustion engines. The exhaust system includes a first diverging section which extends into a cylindrical envelope. A streamlined conical member is axially mounted within the cylindrical envelope and adapted to be moveable in relationship to the engine rating.
U.S. Pat. No. 4,348,862 issued on Sep. 14, 1982 to Fujikawa et al. discloses an exhaust system for a two cycle engine which includes a second expansion chamber having a diffuser in pneumatic connection with a first expansion chamber. The second expanding chamber creates a second pressure drop substantially adjacent the point of pneumatic connection between the first and second expansion chambers. This second pressure drop acts to draw exhaust gases present at the conically divergent section of the first expansion chamber's diffuser.
U.S. Pat. No. 5,012,642 issued on May 7, 1991 to Laimbock discloses an exhaust system for two stroke internal combustion engines which includes a diffuser adapted to be connected to the engine. A baffle is provided downstream of the diffuser and a catalytic converter is disposed between the diffuser and the baffle. The system also includes a diffuser discharge pipe which extends into the diffuser and is much smaller in diameter than the larger diameter of the exhaust system. The diffuser discharge pipe extends from a location adjacent the exhaust gas outlet of the cylinder and the inlet of the diffuser, and merges into the catalytic converter by means of a steep funnel. This ensures that the catalytic converter will be lighted quickly and that gas dynamics will not be adversely affected.
U.S. Pat. No. 5,044,159 issued on Sep. 3, 1991 to Landfahrer et al. discloses an exhaust system for two stroke engines having an exhaust pipe followed by two or more conical sections. The exhaust pipe is of constant cross-section while the conical sections are configured as diffusers having increasing cross-sections. The conical sections are connected to the front end of the exhaust pipe by means of branch pipes of different length. Using this arrangement, a pressure wave coming from the cylinder is twice reflected as a suction wave back to the cylinder. Thus the effective duration of the suction wave is prolonged over that of comparable exhaust systems.
U.S. Pat. No. 5,079,918 issued on Jan. 14, 1992 to Stull discloses an expansion tube exhaust system for internal combustion engines. The system includes a short diverging section connected to a much longer converging section. The longer converging section absorbs negative pressure returning to the engine, thus reducing back pressure to the cylinder. This system is particularly useful in four stroke engines.
Japanese Patent No. 54-17417 and 54-20217, both issued on February 1979, disclose various exhaust systems which incorporate conical sections designed to improve engine output in the high speed region by having a pressure wave reflected from the exhaust system.
East German Patent No. 275,719 issued on September 1988 discloses an exhaust system which incorporates various designs of conical sections to improve output.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.